characterization of the fracture behavior of hydrogels

XIIIth Youth Symposium on Experimental Solid Mechanics, June 29th – July 2nd , Dˇecˇ´ın, Czech Republic
CHARACTERIZATION OF THE FRACTURE BEHAVIOR OF HYDROGELS
J. Sailer 1 , K. Solti 2 , I. Kallai 3 , Z. Major1
Summary: Hydrogels are attractive materials for many demanding applications (i.e. medical, scaffolds for soft tissue engineering, organic electronics and optics). One of the recent
developments in this field is the fabrication of super tough hydrogels by using several modifications of the double-network concept. These hydrogels can show several hundred percent
strains without failure under uniaxial tensile loading condition and reveal excellent fracture
toughness and notch sensitivity. The measurements have been done with two different types
of nanoparticle filled hydrogels which also showed a variation on the percentage of added
nanoclay over a range of 2% to 15%.
Keywords: fracture, displacement controlled loading, crack tip blunting
1 Introduction
Standard hydrogels consist of hydrophilic polymer chains which form a network and can absorb high amounts
of water (over 90%). Besides these common hydrogels, at least three different major types of ultra strength
hydrogels exist First, there are polyacrylamide/alginate hybrid hydrogels which consist of ionically crosslinked alginate chains interpenetrated with the covalently cross-linked polyacrylamide [1]. Furthermore,
there are double-network hydrogels where the minor component is tightly cross-linked polyelectrolytes and
the major component comprises of poorly cross-linked neutral polymers such as polyacrylamide [2]. The last
type are nanocomposite hydrogels which contain a variety of clay minerals with layered crystal structures
that is easily exfoliated in water and act as a cross-linker [3].
2 Preparation and Setup
Pure shear specimens were produced by casting in combined Al and PTFE forms and then conditioned either
as dry or in water, under constant temperature conditions before testing. These specimens were tested under
monotonic, semi-cyclic and cyclic loading conditions using a BOSE Electroforce TestBench testing system
including an extra sensitive 20 N load cell, a temperature chamber and a mounting system with different
clamp sizes. Latter are necessary due to the varying swell of the hydrogels with nanoclay content.
3 Testing methods
3.1 Hysteresis tensile test
We assumed a strong dependence of mechanical properties on the nanoclay content. Hence, two different
modifications of the nanocomposite hydrogels, termed as XLS (sole grade laponite gel) and XLG (gel grade
laponite gel) have been prepared over a range of 2% to 15% added nanoclay. Stress- strain curves for the
XLG grade hydrogels with varying nanoclay content are shown in figure 1.
1
Institute of Polymer Product Engineering, Johannes Kepler University, Linz, Austria, Altenberger Str.
69
email: [email protected]
2
Institute Soft Matters Group, Department of Physical Chemistry and Materials Science, Budapest University of Technology and
Economics of Budapest, H-1521 Budapest, Hungary
3
Linz Center of Mechatronics GmbH, Linz, Austria
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XIIIth Youth Symposium on Experimental Solid Mechanics, June 29th – July 2nd , Dˇecˇ´ın, Czech Republic
Figure 1: Stress-strain curves of XLG nanoparticle
filled hydrogels with varying nanoclay content under dry test conditions.
Figure 2: Stress-strain curves of XLS nanoparticle
filled hydrogels with varying nanoclay content under dry test conditions.
It is also shown that these specimens have been tested under displacement control up to three different
strain levels, namely 100 %, 200 % and 300 % and then stretched up to break. One can see that the degree of
added nanoclay is proportional to degree of hysteresis and mechanical properties such as Young’s modulus
and tensile strength but there is no clear relationship between the nanoclay percentage and fracture strain.
With increasing nanoclay content the hydrogels are stiffer and reveal higher fracture strength.
Figure 3: Impact of nanoclay concentration on tensile strength for both, XLS and XLG.
In the case of XLS we observed similar mechanical properties regarding the added nanoclay content. The
big difference in contrast to the XLG gels is the 25 − 35 % lower fracture strength (always compared to the
XLG gel with equal amount of nanoclay) but significantly higher fracture strain values (XLG from 500 to
1000 % and XLS is from 1000 to 1500 %). The dependence of the tensile strength on the nanoclay content
both for XLG and XLS grades is shown in figure 3.
While XLG grades revealed higher tensile strength with a distinct data scatter, the XLS grades shown
lower tensile strength values and low data scatter.
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XIIIth Youth Symposium on Experimental Solid Mechanics, June 29th – July 2nd , Dˇecˇ´ın, Czech Republic
3.2 Comparison of ”dry” hydrogels vs. hydrogels stored 24h in water
Furthermore, hydrogel specimens from the same batches have been stored 24 hours in water under constant
temperature conditions and the previous tests have been repeated. A clear reduction of the stress level and an
even more significant decrease of tensile strength just due to the 24h storage in water is shown in figure 4.
Moreover, a nanoclay content dependent decrease of the tensile strength was observed (20 % to 350 %) and
this tendency is shown in figure 5. In spite of this high data scatter, a clear trend was noticeable, that is, the
wet hydrogels revealed significantly lower tensile strength.
Figure 4: Dry-wet comparison for XLG4.
Figure 5: Dry-wet comparison for tensile strength.
3.3 Notched hydrogels
The pure shear specimens were subsequently notched in the next test series with a razor blade up to 3 mm
length (1/10 of total length) and the same tests were performed as before. In figure 6, stress- strain curves of
the notched specimens are shown for XLS under dry test conditions.
Figure 6: Stress-strain curves of 3 mm notched XLS nanoparticle filled hydrogels under dry test conditions.
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XIIIth Youth Symposium on Experimental Solid Mechanics, June 29th – July 2nd , Dˇecˇ´ın, Czech Republic
The results are surprising in the way, that tensile strength for notched XLS hydrogels nearly remained at
the same level for every nanoclay variation of the gel. Similar results were observed for XLG hydrogels too.
Hence, all types of hydrogels exhibit really good notch sensitivity.
4 Outlook
One part of future work is focusing on fatigue in general and fatigue crack growth propagation, while another
part will be the study of self-healing properties and property changes due to different temperature regions.
References
[1] JJeong-Yun Sun, Xuanhe Zhao, Widusha R. K. Illeperuma, Ovijit Chaudhuri, Kyu Hwan Oh, David J.
Mooney, Joost J. Vlassak, Zhigang Suo Highly, stretchable and tough hydrogels, Nature 489, 133-136,
2012
[2] Md. Anamul Haque, Takayuki Kurokawa, Jian Ping Gong, Super tough double network hydrogels and
their application as biomaterials Polymer 53, 1805-1822, 2012
[3] Kazutoshi Haraguchi, Toru Takehisa, Simon Fan, Effects of clay content on the properties of nanocomposite hydrogels composed of poly(N -isopropylacrylamide) and clay, Macromolecules, 35, 1016210171, 2002
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